In the processing of microelectronic devices, such as those including semiconductor wafers and other microelectronic devices at any of various stages of processing, substrate surface cleanliness is becoming more and more critical in virtually all processing aspects. Surface cleanliness is measured in many ways and looks at particle presence and/or water marks as contaminants that may affect production of a microelectronic device.
Microelectronic devices include, as examples, semiconductor wafers at any stage of processing and devices such as flat panel displays, micro-electrical-mechanical-systems (MEMS), advanced electrical interconnect systems, optical components and devices, components of mass data storage devices (disk drives), and the like. In general, reduction in the quantity of smaller and smaller particles from such substrate surfaces is desired in order to maximize productivity of devices from semiconductor wafers and to meet quality standards as determined for such devices while doing so with effective and efficient processing steps. Whereas substrate features and surface characteristics, like a hydrophobic or hydrophilic nature of surfaces, affect the rinsing and particle reduction effectiveness, obtaining acceptable substrates of one type does not necessarily predict effectiveness on another. In particular, bare hydrophobic substrates have been found to be increasingly difficult to effectively reduce the presence of smaller and smaller particles because it is difficult to rinse and dry such substrates without adding significant particles.
Substrate surface cleanliness is important at all processing stages after the application of any processing, cleaning or rinsing fluid to a substrate surface, such as by spray dispensing or immersion techniques. After any rinse step, and in particular, after a final rinse step, it is of greater criticality to provide a clean substrate surface with process, cleaning and rinsing fluids and particles effectively removed as determined for any such microelectronic processing. The effectiveness of cleaning a surface with respect to the presence of particle contaminants is typically determined by the ability to measure particles of a predetermined size and greater as are present on the substrate surface after rinsing and drying, for example. As noted above, the trend in the industry is to reduce the presence of smaller and smaller particles from microelectronic devices for greater productivity and device efficiencies. Many techniques have been developed and accepted for effectively determining the quantity of particles on a microelectronic device surface. The effectiveness of cleaning a surface may also be determined with respect to the presence of liquid films and water marks by measuring liquid thicknesses on the surface after rinsing and drying. Techniques for measuring such film thicknesses have also been developed.
Representative steps in wet processing of microelectronic devices include microelectronic device etching, rinsing and drying. As used herein, wet processing includes immersion processing where at least a portion of a microelectronic device is subjected to immersion for a desired period of time and spray processing where process fluids (including rinse fluid) are dispensed to a device surface. Microelectronic device processing typically includes a series of discrete steps such as including a cleaning and/or wet etching step followed by rinsing and drying. These steps may involve the application of a suitable treatment chemical to the substrate surface, e.g., a gaseous or liquid cleaning solution or an etching or oxidizing agent. Such cleaning solutions or etching or oxidizing agents are then preferably removed by a subsequent rinsing step that utilizes a rinsing fluid such as deionized water (DI water) to dilute and ultimately wash away the previously-applied substances. The removal of native oxides on silicon surfaces by sufficient etching typically changes the silicon surface from hydrophilic and renders such HF last-etched surfaces as hydrophobic.
In the case of immersion processing, lifting one or more substrates from a rinse bath (such as a cascade type rinser, as are well known) or lowering the liquid within the vessel can be conducted after the device(s) are adequately rinsed in order to separate the device(s) from the rinse liquid. For spray processing, rinse fluid is dispensed onto a device surface for a determined period while and/or after which a device (or plurality of devices on a carousel in a stack) is rotated or spun at an effective speed to sling the rinse fluid from the device surface. In either immersion or spray processing, it is a goal of such rinsing processes to effectively dry a processed device, i.e. to physically remove as much rinse fluid as possible, in order to reduce the amount of fluid that is left after rinsing to be evaporated from the device surface. Evaporation of rinse fluid may leave behind any contaminants or particles that had been suspended within the fluid.
For enhanced separation or removal of rinse fluid from microelectronic devices after a rinsing step, techniques have been developed to introduce certain compounds that create a surface tension gradient within the rinse fluid at and near the point of separation of the fluid from the device surface. The effect of this, commonly called the Marangoni effect, is to enhance the ability of the rinse fluid (typically DI water) to shed from the device surface under the action of either separating a device from a liquid bath in immersion separation or spinning a device in the case of spray dispensing. The removal of rinse fluid has been found to be enhanced on either hydrophilic or hydrophobic device surfaces with such techniques. Compounds that affect surface tension and create such a surface tension gradient are known and include isopropyl alcohol (IPA), 1-methoxy-2-propanol, di-acetone alcohol, and ethyleneglycol and are hereinafter referred to as tensioactive compounds. See for example, U.S. Pat. No. 5,571,337 to Mohindra et al. for an immersion type vessel and U.S. Pat. No. 5,271,774 to Leenaars et al. for a spin dispensing apparatus, each of which utilize the Marangoni effect as part of the removal of rinse fluid.
After rinsing, a thin film of rinse fluid may remain on some or all of a microelectronic device surface (particularly with a hydrophilic surface) and/or liquid drops may remain at certain points, such as are known to sometimes form at edge gripper contact points. Any such remaining fluid is desirably removed with a further drying step. The rinsing and drying steps are, in general, separate processing events. Drying does not typically begin until the substrate surface has been rinsed as completely as possible of contaminants and processing chemicals. A particular drying operation utilized depends on parameters of the separation or removal (e.g. speed of separation or spinning, orientation of the microelectronic devices, and the like) as well as characteristics of the microelectronic devices themselves (e.g. the hydrophilic or hydrophobic nature of the device surface, the presence of patterning or not on the device surface). Any liquid droplets or films that remain on a microelectronic device surface after rinsing and separation are desirably removed from the microelectronic device surface. If such droplets or films are left to evaporate from the microelectronic device surface, any contaminants suspended within the droplets or films might be deposited on the microelectronic device surface, which contaminants may render a portion of the microelectronic device unsuitable for further processing or use. Known drying techniques include the use of heated gases, such as heated nitrogen gas, after the rinsing step for removing unwanted droplets and films from the microelectronic device surfaces.
One important aspect in providing clean microelectronic devices after wet processing is to start with the use of clean processing liquids. Clean liquid use can be controlled by known or developed filtering processes so as to minimize introduction of contaminants into the processing environment. This is particularly true where devices are being cleaned or rinsed by a wet process, such as by using DI water as a rinse liquid. Specific filtering techniques for ultra-clean DI water have been developed for use in the microelectronic industry, such as those described in U.S. Pat. Nos. 5,542,441, 5,651,379 and 6,312,597 to Mohindra et al.
Microelectronic devices are often rinsed as a batch within an immersion vessel (such as maintained in a spaced orientation by a cassette or holder) or within a spray processor (such as provided on a carousel). More recently, there has been greater interest in the development of single wafer wet processing. For immersion single wafer processing, smaller single wafer vessels have been developed as described in copending U.S. patent application Ser. No. 10/243,616 to Christenson et al. Spray processing, however, fits well with the concept of single wafer processing because of the orientation of such microelectronic devices in a horizontal fashion and the easier loading and unloading of such devices as provided to and from other processing stations. Also, there is a greater potential for quicker throughput of single devices with spray processing.
However, due to the horizontal nature of the treated surface of the microelectronic device as may be processed within a spray processor, it is more difficult to obtain a clean removal of processing fluids and reduction in contaminants from the device surface. Thus, even after a rinse step, unacceptable levels of contaminants or particles can be present for a particular application especially as device features become smaller. With hydrophobic surfaces, such as result from an HF last-etched silicon surface, small particle count reduction is found to be most difficult. An important aspect in particle count reduction is to rinse and dry a device while minimizing particles left behind.
An attempt to obtain substrates with better removal of processing fluids from horizontally rotated substrates is described in U.S. Pat. No. 6,568,408 to Mertens et al. Described are methods and equipment that controllably create a sharply defined liquid-vapor boundary, which boundary is moved across the substrate surface along with moving liquid and vapor delivery nozzles. As described in the Mertens et al patent, a surface tension gradient is theoretically created within such boundary by the specific delivery of the vapor to the boundary as such is miscible within the liquid for enhancing liquid removal based upon the Marangoni effect. Such a system may be more effective on hydrophilic surfaces, but adds significantly to the complexity of the system and the manner of control needed to obtain rinsing with adequate rinse fluid removal. The effectiveness of such a system is significantly less for completely hydrophobic surfaces, such as HF last-etched silicon wafers, where a reduction in contaminants, such as small particles, is still desired.
The Leenaars et al U.S. Pat. No. 5,271,774, noted above, describes an apparatus and methods for delivering organic solvent vapor to a substrate surface after it is rinsed and leaves a water film layer on the substrate surface (as such naturally forms on a hydrophilic wafer surface) followed by rotation. Organic solvent vapor is introduced into a process chamber, preferably unsaturated, as controlled by the vapor temperature. FIGS. 2, 3 and 5 show the sequence of starting with a rinse water film on a substrate surface followed by the film's breaking up into thicker drops as a result of exposure to the organic solvent vapor. Then, the drops are more easily slung from the surface by rotation. Whereas the action of the organic solvent vapor is to create drops from a film of water as such a film layer is possibly provided on a hydrophilic surface, such action would not be required in the situation where a hydrophobic surface is rinsed with water since the same effect is naturally created. For a hydrophobic surface, the rinse water beads into drops on the device surface due to the nature of the surface. Again, there is a need to improve the reduction of contaminants on all surfaces, but in particular, for hydrophobic device surfaces.